Incorrect vector direction for the surfaces is a very common error or warning in EPlus IDF.
Your observation is correct that "you determine the upper-left corner by standing in front of the surface and look "into" the zone". You are standing outside the zone. This is how you define the order in the IDF.
I am an engineer and am used to the idea of right-hand rule to relate the directions of E-M fields and current, etc. The need for the fixed ordering of the vertices is to specify a vector direction of the surface.
Anticlockwise ordering will specify a vector pointing out of the paper, as the z-axis. When you apply the right hand rule, you let the fingers follow the ordering, the thumb is the direction of the vector. Same as the right-hand rule defining the x, y, z coordinate directions.
Following the anticlockwise rule viewing from outside, all the walls of a zone will have the vectors pointing outwards. You will need to view the floor from below, and the roof or ceiling from above. Using the right hand rule, you need not place yourself in or out side the zone, and just observe which direction the thumb is pointing.
The vector directions are listed in the HTML output file.
If a ceiling is also the floor of the upstairs, the floor need the ordering reversed.
If the ceiling has the vertices in 1,2,3,4 order, you will need to change the floor to 1, 4,3, 2 ordering. If you try the right hand rule, the ceiling has the vector upwards, and floor has the vector downwards.
In general, a zone must have all its surface vectors pointing outwards.
Fixing the Starting point (Upper Left corner) will make it easier to generate a image surface, for shading and reflection calculation. That may be the reason why it is not so important for the floors and ceilings.
Dr. Li
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: SX99A@xxxxxxxxx
Date: Wed, 22 Jul 2009 07:28:06 +0000
Subject: [EnergyPlus_Support] How to determine the "upper-left" corner of a horizontal surface?
When using "surface" to create a building model, EP requires the input of the coordinates of each corner. The rule is such that you start from the "upper-left" corner and proceed counter-clockwise. For vertical surfaces like walls, you determine the upper-left corner by standing in front of the surface and look "into" the zone. This way, the "upper-left" corner is clearly defined.
However, I am a little confused by using this rule on a horizonal surface, say a roof or a slab. Just looking "into" the zone is not enough to determine the "upper-left" corner because you can rotate to North, South, East, or West.
I would surely appreciate if you can kindly answer my question.
Have a good day!
However, I am a little confused by using this rule on a horizonal surface, say a roof or a slab. Just looking "into" the zone is not enough to determine the "upper-left" corner because you can rotate to North, South, East, or West.
I would surely appreciate if you can kindly answer my question.
Have a good day!
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